1. Field of the Invention
A process is provided for separating hafnium from zirconium in a solution prepared from zirconium oxychloride.
2 Description of the Related Art
Commercial scale separation of Zr and Hf traditionally involves a liquidxe2x80x94liquid extraction process, wherein hafnium is extracted from an aqueous phase containing mixed tetrachlorides into an organic phase. The ZrCl4 and contaminating HfCl4 are derived from the carbochlorination of zircon (ZrSiO4), in which the Hf is a naturally-occurring impurity. Hafnium typically is present in such mixtures to the extent of about 2% by weight (wt.) of the Zr. Processes for separating Hf from Zr are described in, for example, U.S. Bureau of Mines (USBM) Report of Investigations 5499 (1959), entitled xe2x80x9cZirconium-Hafnium Separationxe2x80x9d (xe2x80x9cUSBM 5499xe2x80x9d), and U.S. Pat. Nos. 2,938,769; 2,952,513; 3,006,719; 3,069,232; 4,202,862; and 5,160,482 and the references cited therein, the disclosures of all of these patents being incorporated herein by reference in their entirety.
The typical process practiced commercially today is much as described in USBM 5499. That document, however, devotes little space to the difficulties in deriving the mixed chlorides of Zr and Hf. In U.S. Pat. No. 5,160,482, more of these difficulties are recounted. One primary difficulty is that commercially available zirconium tetrachloride ((Zr+Hf)Cl4) typically contains contaminants such as Fe, P, Al, Ra, Th and U which are removed by additional sub-processes. Typical commercially-available (Zr+Hf)Cl4 preparations include iron content, which must be removed prior to extraction of the Hf content because Fe often causes polymerization of the organic constituents of the separation mixture. Therefore, there is need for an improved process that can obtain the products of separation, the respective pure oxides ZrO2 and HfO2.
Briefly, in the presently practiced art, the mixed tetrachlorides of Zr and Hf are dissolved in water to yield a strongly acidic aqueous solution. To this solution is added a solution of ammonium thiocyanate, and this combined solution is countercurrently contacted in extraction columns with a solution of thiocyanate in the substantially water-immiscible solvent, methyl isobutyl ketone (MIBK). This contact occurs in large columns involving many stages of interphase mass transfer, over which the Hf preferentially reports to the MIBK (organic) phase, while the Zr, remaining in the aqueous phase, gradually is enriched so that the aqueous phase contains less than the 100 ppm wt. Hf/(Hf+Zr) required for use in nuclear reactors. At the same time, in the organic phase, the Hf is enriched to about 98%. The Zr and Hf xe2x80x9craffinatexe2x80x9d streams, are then processed by methods described in the art to recover ZrO2 with  less than 100 ppm wt. Hf/(Hf+Zr) with  less than 2% wt. Zr. These extractions typically take place in multiple columns, substantially as shown in USBM 5499 (FIG. 3) and in U.S. Pat. No. 2,938,769 (FIG. 2).
U.S. Pat. Nos. 2,938,769 and 3,006,719 each disclose separation of Zr and Hf substantially as described above, but using zirconium oxychloride (Zr+Hf)OCl2, commercially available as (Zr+Hf)OCl2xc2x78H2O crystals, as a feed material, which contains contaminating HfOCl2 at similar levels as found in commercially available pre-separation (Zr+Hf)Cl4. Use of (Zr+Hf)OCl2, is advantageous because there are less heavy metal contaminants in the commercially-available (Zr+Hf)OCl2 as compared to commercial preparations of (Zr+Hf)Cl4. U.S. Pat. Nos. 2,938,769 and 3,006,719 describe mixing (Zr+Hf)OCl2 feed stock with a thiocyanate salt, typically NH4SCN, and certain quantities of HCl. The feed stock is contacted with an organic phase containing thiocyanic acid, into which the Hf partitions. Although this extraction method often works well, the extraction of Hf from the aqueous phase to produce an aqueous Zr-containing raffinate stream, is fraught with inconsistency. For commercial viability, the Hf must be removed from the aqueous phase so that levels of Hf less than 100 ppm wt. Hf/(Hf+Zr) in the aqueous Zr-containing raffinate stream are realized consistently.
In recognition that the acidity of the (Zr+Hf)OCl2 feed stock affects the separation of Hf+Zr, provided is an improved method for separating Zr species (ZrOCl2 and ZrCl4) and Hf species (HfOCl2 and HfCl4) from a feed stock prepared from (Zr+Hf)OCl2xc2x78H2O crystals. The method recognizes that the ratio of total acid in the feed stock to total metal Zr+Hf species must be maintained at certain levels to achieve system stability for the Hf extraction process.
Described herein is a method for separating hafnium and zirconium, and, alternatively to optimize separation thereof. The method includes the steps of extracting an aqueous feed stock comprising zirconium oxychloride and hafnium oxychloride and a thiocyanate salt with a suitable thiocyanate-containing organic solvent to produce a zirconium-containing aqueous raffinate stream and a hafnium-containing organic raffinate stream. In the method, the TA/MO2 ratio (the ratio of total acidity (moles/L) to metal oxide (Zr+Hf)O2 (moles/L), in calcined feed stock) of the aqueous feed stock is maintained in a range of from greater than about 2.55 to less than about 3.5, with a ratio of about 2.75 being a typical target ratio. The extraction step may be conducted in, for example, multiple columns by contacting the aqueous feedstock with a countercurrent stream of the organic solvent. A method of preparing the above-described feedstock also is provided.